1. Technical Field
Embodiments of the subject matter disclosed herein generally relate to devices and methods for transmitting backup emergency disconnect signals (EDS) to subsea pods configured to control blowout preventers.
2. Discussion of the Background
Oil and gas extraction remains a critical component of the world economy in spite of increasing challenges regarding the accessibility of the oil reserves and the safety of the exploitation. Thus, drilling at offshore locations to extract oil and gas from under the sea floor is widely used worldwide. Subsea oil and gas exploration becomes even more challenging as sea depth at the well locations increases.
An offshore oil and gas installation 1 includes a subsea blowout preventer stack useable to seal a wellhead for safety and environmental reasons. As shown in FIG. 1, the subsea blowout preventer stack includes a lower blowout preventer (“BOP”) stack 10 attached to a wellhead on the sea floor 20, and a Lower Marine Riser Package (“LMRP”) 30, which is attached to a distal end of a drill string 40. The drill string 40 extends from a drill ship 50 (or any other type of surface drilling platform or vessel) towards the wellhead. During regular operation the lower BOP stack 10 and the LMRP 30 are connected. At times, blowout preventers 25 located in the lower BOP stack 10 and in the LMRP 30 may be closed. The LMRP 30 may be disconnected from the lower BOP stack 10 and retrieved to the surface, leaving the lower BOP stack 10 atop the wellhead, on the sea floor 20. Thus, for example, the LMRP 30 may be disconnected and retrieved when inclement weather is expected or when use of the wellhead is temporarily stopped.
Electrical cables and/or hydraulic lines 60 transport control signals from the surface (i.e., the drill ship 50) to two pods 70 and 75 which are part of the LMRP 30. The two pods 70 and 75 control the BOPs and other devices in the LMRP 30 and the lower BOP stack 10 according to signals received from the surface (i.e., the drill ship 50). The two pods 70 and 75, known as the “yellow” pod and the “blue” pod are substantially identical and ensure redundancy (i.e., if one fails, the other takes over).
Upon receiving an EDS signal, the pod(s) 70 and/or 75 control closing of the BOPs of the LMRP 30 and the lower BOP stack 10. However, the EDS signal may not reach the pods 70 and 75 when the electrical cables 60 are unintentionally interrupted. In order to receive the control signals at the pod(s), physical continuity of the electrical cables 60 is necessary. However, the electrical cables 60 may have been interrupted accidentally when an emergency situation triggering the necessity to send an EDS signal from the surface to the pod(s) 70 and/or 75 occurs. If the EDS signal does not reach the pod(s) 70 and/or 75 and the BOPs are not closed, the consequences may be dire for the operating personnel, the equipment and the environment.
In some installations, an acoustic backup EDS signal may be transmitted acoustically via the sea water. However, when a distance between the water surface and the LMRP is large, this acoustic backup EDS signal may be dumped and lost. In addition environmental interference in the sea water due to the emergency situation occurring could prevent the acoustic backup EDS signal from being received properly subsea.
Accordingly, it would be desirable to provide a backup transmission of the EDS signal from the surface to pods attached to the LMRP, using another path than the electrical cables and/or water column.